Pollinators are important drivers for angiosperm evolution and diversification, but it is not well understood how changes in the pollinator assemblage can affect plant evolution in the short term. To address this question, my thesis investigates the consequences for plants of the addition and removal of floral visitors for two flowering species, Digitalis purpurea and Nicotiana glauca, both of which recently colonised new continents and show variation in pollinators between their native and introduced ranges. In D. purpurea, I found that populations in the introduced range pollinated by a new functional group (hummingbirds) had larger proximal corolla tubes than in the native range. Together with directional selection observed for the trait in the introduced range, I demonstrate rapid evolution of proximal corolla tube traits following range expansion. Floral traits in this species show high heritabilities in all populations, in line with the ability to evolve rapidly. I also demonstrate that the addition of nectar robbers to D. purpurea plants, which are present in the introduced populations only, can cause significant negative effects on the female component of reproduction. In N. glauca, a plant pollinated by hummingbirds in the native range but self-pollinated in the introduced range, I found a relationship between stigma-anther distance and the rate of self-pollination in flowers but not at the level of the plant. I observed a high degree of within-individual variability for stigma-anther distances which may limit the response to selection for shorter stigma-anther distances to increase the rate of selfing. In this thesis, I show that the addition and removal of pollinators to the floral visitor assemblage of a plant can have consequences for floral trait evolution
